556 PALAEONTOLOGY OF ILLINOIS: 



strules very small, 0.25 by 1.2 mm., sub-oval, eleven or twelve 

 in 1 cm. Carina about 0.4 mm. in height, thin at first, with 

 the summit abruptly expanded and angular along the middle. 

 The sides are wavy, the undulations corresponding closely both in 

 number and position, with the zooecia apertures beneath them. 

 Zooecia in two ranges, one on each side of the median keel. 

 Apertures circular, opening directly upward, about 0.1 mm. in 

 diameter, as much or a little more apart, eighteen in 5 mm.; 

 with well developed peristome. On the reverse the fenestrules 

 vary from quite regularly circular to broad oval, are 0.3 or 

 0.4 mm. in diameter, and much farther apart longitudinally 

 than transversely. Surface of branches and dissepiments, when 

 well preserved, with numerous small nodes, and varying between 

 slightly convex or flat and appreciably concave. In the last 

 case the fenestrules are encircled by an elevated rim. 



This species is smaller in its proportions than either Fene- 

 stella semirotunda or F. permarginata Hall, from the same 

 locality. Both of those species, which clearly belong to Semi- 

 coscinium, also have twenty cells to 5 mm., while in this there 

 are only eighteen in that distance. Right here it may be well 

 to remark that Hall's figures of the two species mentioned 

 (Pal. N. Y. Yol. VI, PI. 49) appear to be somewhat mixed. 



Position and locality: Upper Helderberg group; Falls of the 

 Ohio river. 



SEMICOSCINIUM RHOMBICUM Ulrich. 



PI. XLIV, figs. 4-4a, and PI. LIV.fig. 8. 



Zoarium infundibuliform, large, undulated. Celluliferous face 

 with the branches rather slender, more or less flexuous, alter- 

 nately approaching and receding from each other, bifurcating 

 at long intervals, very gradually increasing from 0.3 to 0.5 

 mm. between the bifurcations, seventeen or eighteen to 1 cm. 

 Dissepiments very short, the branches appearing almost to in- 

 osculate, their width equal to or greater than the length of the 

 fenestrule, varying from 0.6 to 1.0 mm. Fenestrules oval, 

 rather unequal, but averaging about 0.6 by 0.2 mm., eight in 

 1 cm.; regularly alternating in the adjoining longitudinal series. 



